Walker, now a pundit, claimed Celtic's Champions League campaign "ended with a whimper" as they lost 4-2 on aggregate to Braga in the third qualifying round and he also criticised Lennon's transfer record.

Lennon reacted angrily, claiming Walker had been "economical with the truth" about transfers into Celtic as well as using various statistics to suggest the 2-1 second-leg win over Braga at Parkhead on Wednesday was merited.

However, Walker, in his Sunday Mail column, refused to back down and added further criticism of the Celtic board.

He said: "If Neil Lennon takes the time to snap back at all the criticism of Celtic this season he's in for a torrid campaign. I stand by all the comments that annoyed the Hoops boss after the Euro defeat by Braga.

"Any team that is 4-0 down on aggregate with just over 30 minutes left of the tie has gone out with a whimper. If Neil wants to deal in stats, there wasn't a single Celtic shot on target in the first half.

"Unfortunately, Neil missed the point I was trying to make about the club's signing policy. Recruiting players from Mexico, Korea and Israel to tap into the potential commercial spin-offs doesn't give him the priority when it comes to signings.

He added: "As for the criticism of Celtic skipper Scott Brown? In my book he has been a huge disappointment since signing. Brown hasn't influenced enough big games to justify his £4.5 million fee or reputation.

"Criticism is part of football, especially at Celtic, and my general point was about boardroom interference on too many football issues at Parkhead. That was summed up by the club's ridiculous pursuit of Finland coach Stuart Baxter.

"Here is a man who was linked with every job in Scotland for more than a decade and eventually became a joke figure. Yet somehow the Parkhead board thought it would be a good idea to bring him in as a part-time adviser to Lennon.

"The Hoops boss was made to look foolish by the club when he announced Baxter would be a part-time consultant. Just 24 hours later the furious Finnish FA denied any agreement was in place and Celtic were forced to change their story."